Electroconductive thick film composition(s), electrode(s), and semiconductor device(s) formed therefrom

ABSTRACT

The present invention is directed to an electroconductive thick film composition comprising: (a) electroconductive metal particles selected from (1) Al, Cu, Au, Ag, Pd and Pt; (2) alloy of Al, Cu, Au, Ag, Pd and Pt; and (3) mixtures thereof; (3) glass frit wherein said glass frit is Pb-free; dispersed in (d) an organic medium, and wherein the average diameter of said electroconductive metal particles is in the range of 0.5-10.0 μm. The present invention is further directed to an electrode formed from the composition as detailed above and a semiconductor device(s) (for example, a solar cell) comprising said electrode.

FIELD OF THE INVENTION

The present invention is directed primarily to thick film compositions,electrodes, and semiconductor devices. It is further directed to asilicon semiconductor device. In particular, it pertains to anelectroconductive composition used in the formation of a thick filmelectrode of a solar cell. The present invention is further directed toa silver electroconductive thick film composition (paste) for a solarcell.

TECHNICAL BACKGROUND OF THE INVENTION

The present invention can be applied to a broad range of semiconductordevices, although it is especially effective in light-receiving elementssuch as photodiodes and solar cells. The background of the invention isdescribed below with reference to solar cells as a specific example ofthe prior art.

A conventional solar cell structure with a p-type base has a negativeelectrode that is typically on the front-side or sun side of the celland a positive electrode on the back side. It is well known thatradiation of an appropriate wavelength falling on a p-n junction of asemiconductor body serves as a source of external energy to generatehole-electron pairs in that body. Because of the potential differencewhich exists at a p-n junction, holes and electrons move across thejunction in opposite directions and thereby give rise to flow of anelectric current that is capable of delivering power to an externalcircuit. Most solar cells are in the form of a silicon wafer that hasbeen metallized, i.e., provided with metal contacts that areelectrically conductive.

Most electric power-generating solar cells currently used on earth aresilicon solar cells. Process flow in mass production is generally aimedat achieving maximum simplification and minimizing manufacturing costs.Electrodes in particular are made by using a method such as screenprinting from a metal paste.

An example of this method of production is described below inconjunction with FIG. 1.

FIG. 1 shows a p-type silicon substrate, 10.

In FIG. 1( b), an n-type diffusion layer, 20, of the reverseconductivity type is formed by the thermal diffusion of phosphorus (P)or the like. Phosphorus oxychloride (POCl₃) is commonly used as thephosphorus diffusion source. In the absence of any particularmodification, the diffusion layer, 20, is formed over the entire surfaceof the silicon substrate, 10. This diffusion layer has a sheetresistivity on the order of several tens of ohms per square (Ω/□), and athickness of about 0.3 to 0.5 μm.

After protecting the front surface of this diffusion layer with a resistor the like, as shown in FIG. 1( c), the diffusion layer, 20, is removedfrom the rest of the surfaces by etching so that it remains only on thefront surface. The resist is then removed using an organic solvent orthe like.

Next, a silicon nitride film, 30, is formed as an anti-reflectioncoating on the n-type diffusion layer, 20, to a thickness of about 700to 900 Å in the manner shown in FIG. 1( d) by a process, such as plasmachemical vapor deposition (CVD).

As shown in FIG. 1( e), a silver paste, 500, for the front electrode isscreen printed then dried over the silicon nitride film, 30. Inaddition, a back side silver or silver/aluminum paste, 70, and analuminum paste, 60, are then screen printed and successively dried onthe back side of the substrate. Firing is then typically carried out inan infrared furnace at a temperature range of approximately 700 to 950°C. for a period of from several minutes to several tens of minutes.

Consequently, as shown in FIG. 1( f), aluminum diffuses from thealuminum paste into the silicon substrate, 10, as a dopant duringfiring, forming a p+ layer, 40, containing a high concentration ofaluminum dopant. This layer is generally called the back surface field(BSF) layer, and helps to improve the energy conversion efficiency ofthe solar cell.

The aluminum paste is transformed by firing from a dried state, 60, toan aluminum back electrode, 61. The back side silver or silver/aluminumpaste, 70, is fired at the same time, becoming a silver orsilver/aluminum back electrode, 71. During firing, the boundary betweenthe back side aluminum and the back side silver or silver/aluminumassumes an alloy state, and is connected electrically as well. Thealuminum electrode accounts for most areas of the back electrode, owingin part to the need to form a p+ layer, 40. Because soldering to analuminum electrode is impossible, a silver or silver/aluminum backelectrode is formed over portions of the back side (often as 5-6 mm widebusbars) as an electrode for interconnecting solar cells by means ofcopper ribbon or the like. In addition, the front electrode-formingsilver paste, 500, sinters and penetrates through the silicon nitridefilm, 30, during firing, and is thereby able to electrically contact then-type layer, 20. This type of process is generally called “firingthrough.” This fired through state is apparent in layer 501 of FIG. 1(f).

As noted above, the back surface electrode that is used to interconnectthe solar cells through soldering may comprise Ag or Ag/Al compositions.When prior art Ag compositions are used, they can provide goodsolderability and adhesion. However, since the Ag compositions cannotproduce a back surface field, conversion efficiency of the solar cellsuffers. On the other hand, when Ag/Al compositions are used, adhesionstrength is generally lowered and gives rise to concerns regarding longterm reliability. This is due to the fact that the addition of Al willgenerally hurt solderability and thus, adhesion performance.

Furthermore, there is an on-going effort to provide compositions whichare Pb-free while at the same time maintaining electrical performanceand other relevant properties of the device. The present inventorsprovide novel Ag/Al comprising composition(s) and semiconductor deviceswhich simultaneously provide such a Pb-free system while stillmaintaining electrical performance, and improving adhesion. The currentinvention provides such compositions and devices. Furthermore, thecomposition(s) of the present invention lead to reduced bowing in someembodiments of the invention.

SUMMARY OF THE INVENTION

The present invention is directed to an electroconductive thick filmcomposition comprising:

-   -   (a) electroconductive metal particles selected from (1) Al, Cu,        Au, Ag, Pd and Pt; (2) alloy of Al, Cu, Au, Ag, Pd and Pt;        and (3) mixtures thereof;    -   (b) glass frit wherein said glass frit is Pb-free; dispersed in    -   (c) an organic medium, and wherein the average diameter of said        electroconductive metal particles is in the range of 0.5-10.0        μm. The present invention is further directed to an electrode        formed from the composition, as detailed above, and a        semiconductor device comprising said electrode.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a process flow diagram illustrating the fabrication of asemiconductor device. Reference numerals shown in FIG. 1 are explainedbelow.

-   -   10: p-type silicon substrate    -   20: n-type diffusion layer    -   30: silicon nitride film, titanium oxide film, or silicon oxide        film    -   40: p+ layer (back surface field, BSF)    -   60: aluminum paste formed on back side    -   61: aluminum back electrode (obtained by firing back side        aluminum paste)    -   70: silver/aluminum paste formed on back side    -   71: silver/aluminum back electrode (obtained by firing back side        silver/aluminum paste)    -   500: silver paste formed on front side    -   501: silver front electrode (formed by firing front side silver        paste)

FIGS. 2( a)-(d) explain the manufacturing processes for manufacturing asolar cell using the electroconductive paste of the present invention.Reference numerals shown in FIG. 2 are explained below.

-   -   102 Silicon substrate    -   104 Light-receiving surface side electrode    -   106 Paste composition for a first electrode    -   108 Electroconductive paste for a second electrode    -   110 First electrode    -   112 Second electrode

DETAILED DESCRIPTION OF THE INVENTION

The conductive thick film composition (paste) of the present inventionprovides the ability to form an electrode from said paste wherein theelectrode has high adhesion on the Si substrate, high back surfacefield, and low bowing.

In order to achieve the above-mentioned objective, the present inventionis a conductive thick film composition, in particular a silver/aluminumelectroconductive paste, which comprises silver and aluminum particles,glass particles, optional inorganic additives, and an organic vehicle,and is used in an electrode for connecting a back face terminal on thesilicon substrate of a solar cell. It is characterized by the fact thatthe average particle diameter of said silver/aluminum particles is0.5-10.0 μm. In the present invention, preferably, the silver particlesare included at 40-93 wt % based on the total weight of the paste,aluminum is in the 1-5 wt % based on the total weight of the paste, theglass particles are included at 2-0 wt % based on the total weight ofthe paste, the inorganic additive is in the 0-2 wt % based on the totalweight of the paste, and the organic vehicle is included at 5-50 wt %based on the total weight of the paste. Furthermore, it is preferablefor the glass powder included in the silver electroconductive paste tohave a softening point of 300-550° C.

The electroconductive paste of the present invention is used either indirect contact with the Si substrate or printed on top of the Al film.

Each component of the electroconductive thick film paste of the presentinvention is explained in detail below.

1. Electroconductive Metal

In the present invention, the conductive phase is most preferably thesilver(Ag)/aluminum(Al); however, an electroconductive metal(s) otherthan silver/aluminum, for example, Cu, Au, Ag, Pd, Pt, Al, Ag—Pd, Pt—Au,etc., can also be used. In fact, the electroconductive metal particlesmay be selected from (1) Al, Cu, Au, Ag, Pd and Pt; (2) alloy of Al, Cu,Au, Ag, Pd and Pt; and (3) mixtures thereof.

Typically, as the electroconductive metal usable in theelectroconductive paste of the present invention, silver (Ag) andaluminum (Al) will be the conductive phase in the present composition.The silver and aluminum particles are preferably in the flake orspherical (sometimes referred to as powder) form. Silver is used tobenefit its high conductivity and solderability; aluminum is used tobenefit its capability of inducing back surface field and thusconversion efficiencies.

It is preferable for the silver and aluminum to have a high purity(99+%); however, a substance with a low purity can also be used inresponse to the electrical demand of the electrode pattern.

2. Inorganic Binder

It is preferable for the electroconductive paste of the presentinvention to include an inorganic binder. The inorganic binder usable inthe present invention is a glass frit with a preferred softening pointof in the range of 300-550° C., so that the electroconductive paste canbe fired at 700-950° C., appropriately sintered and wetted, andappropriately adhered to a silicon substrate. If the softening point islower than 300° C., the glass may melt early and get to the surface ofthe electrode and interfere with soldering. On the other hand, if thesoftening point is higher than 550° C., the glass will not have enoughtime to soften and wet the conductive phases, and a sufficient adhesivestrength is not exerted, and the liquid-phase sintering of the silversometimes cannot be promoted. It is understood that if the firingtemperature range used in the process is changed, the softening pointtemperature range of the optimal glass frit will also change.

Here, the “softening point” is that obtained by the fiber elongationmethod of ASTM C338-57.

The glass chemistry plays a role in the current invention, not only onthe adhesion strength, but also on the bowing and electricals.Furthermore, the invention is limited to Pb-free glass. Useful glassesin the present invention include bismuth-based glasses. Some typicalglass compositions comprise the following (in weight percent total glasscomposition): SiO2: 0.5-35%; Al2O3: 0-5%; B2O3: 1-15%; ZnO: 0-15%:Bi2O3: 55-90%. Selected glass compositions are listed in Table 1.

TABLE 1 Selected Glass Compositions Glass SiO₂ Al₂O₃ B₂O₃ ZnO Bi₂O₃ Na₂OLi₂O CaO BaO CeO2 ZrO2 TiO2 MgO I 7.11 2.13 8.38 12.0 69.82 0.53 II 1.5014.9 81.50 1.0 1.0 0.1 III 1.0 0.50 9.50 13.0 73.0 3.0 IV 4.18 0.65 9.9312.98 70.81 0.92 0.54 V 32.72 2.90 2.91 48.20 3.10 0.87 2.90 5.23 1.17

The content of the glass frit as the inorganic binder is notparticularly limited as long as it is an amount that can achieve theobjective of the present invention; however, the content is typically1-10 weight percent, preferably 2-6 weight percent, based on the totalweight of the electroconductive paste.

If the amount of the inorganic binder is smaller than 1 weight percent,the adhesive strength is sometimes insufficient and electricalperformance may also be affected, and if the amount of the inorganicbinder is more than 10 weight percent, soldering may become verydifficult. In addition, bowing could be affected adversely as thepercentage of glass content increases.

3. Optional Inorganic Additives

The glass frit (inorganic binders) used in the compositions of thepresent invention provide adhesion, however the total amount ofinorganic glass binder that may be added to the total electroconductivecomposition is limited by bowing and solderability requirements.Therefore additional inorganic additives may, optionally be added toincrease adhesion characteristics. These additional optional additivesmay be selected from (1) TiB2, Cu, Ti, Al, Sn, Sb, Cr, Fe, Mn, Co, Ni,Ru, B and Bi; (2) compounds that can generate elemental metals selectedfrom Cu, Ti, Al, Sn, Sb, Cr, Fe, Mn, Co, Ni, Ru, B and Bi; (3) oxides ofCu, Ti, Al, Sn, Sb, Cr, Fe, Mn, Co, Ni, Ru, B and Bi; and (4) mixturesthereof.

The present inventors have found that small amounts of optionaladditives, such as Cu powder or metal oxides, Bi2O3, TiO2, TiB2, Al2O3,B2O3, SnO2, Sb2O5, Cr2O3, Fe2O3, CuO, Cu2O, MnO2, Co2O3, NiO, RuO2, etc.can help increase adhesion characteristics, without affecting electricalperformance and bowing.

The average diameter of the optional inorganic additives is in the rangeof 0.5-10.0 μm, or dispersed to the molecular level when the additivesare in the form of organo-metallic compounds.

4. Organic Medium

The inorganic components are typically mixed with an organic medium bymechanical mixing to form viscous compositions called “pastes”, havingsuitable consistency and rheology for printing. A wide variety of inertviscous materials can be used as organic medium. The organic medium mustbe one in which the inorganic components are dispersible with anadequate degree of stability. The rheological properties of the mediummust be such that they lend good application properties to thecomposition, including: stable dispersion of solids, appropriateviscosity and thixotropy for screen printing, appropriate wettability ofthe substrate and the paste solids, a good drying rate, and good firingproperties. The organic vehicle used in the thick film composition ofthe present invention is preferably a nonaqueous inert liquid. Use canbe made of any of various organic vehicles, which may or may not containthickeners, stabilizers and/or other common additives. The organicmedium is typically a solution of polymer(s) in solvent(s).Additionally, a small amount of additives, such as surfactants, may be apart of the organic medium. The most frequently used polymer for thispurpose is ethyl cellulose. Other examples of polymers includeethylhydroxyethyl cellulose, wood rosin, mixtures of ethyl cellulose andphenolic resins, polymethacrylates of lower alcohols, and monobutylether of ethylene glycol monoacetate can also be used. The most widelyused solvents found in thick film compositions are ester alcohols andterpenes such as alpha- or beta-terpineol or mixtures thereof with othersolvents such as kerosene, dibutylphthalate, butyl carbitol, butylcarbitol acetate, hexylene glycol and high boiling alcohols and alcoholesters. In addition, volatile liquids for promoting rapid hardeningafter application on the substrate can be included in the vehicle.Various combinations of these and other solvents are formulated toobtain the viscosity and volatility requirements desired.

The polymer present in the organic medium is in the range of 8 weightpercent to 11 weight percent of the total composition. The thick filmcomposition of the present invention may be adjusted to a predetermined,screen-printable viscosity with the organic medium.

The ratio of organic medium in the thick film composition to theinorganic components in the dispersion is dependent on the method ofapplying the paste and the kind of organic medium used, and it can vary.Usually, the dispersion will contain 70-95 wt % of inorganic componentsand 5-30 wt % of organic medium (vehicle) in order to obtain goodwetting.

The electroconductive paste of the present invention is typicallyconveniently manufactured by power mixing, a dispersion technique thatis equivalent to the traditional roll milling. The electroconductivepaste of the present invention is preferably spread on the desired partof the back face of a solar cell by screen printing; in spreading bysuch a method, it is preferable to have a viscosity in a prescribedrange. The viscosity of the electroconductive paste of the presentinvention is preferably 80-200 PaS when it is measured at 10 rpm and 25°C. by a utility cup using a Brookfield HBT viscometer and #14 spindle.

The electroconductive paste of the present invention can be used eitherin direct contact with Si wafers or printed on top of the dried Al film.The Ag/Al film can be cofired with Al at the same time in a processcalled cofiring. Next, an example in which a solar cell is preparedusing the electroconductive paste (silver/aluminum electroconductivepaste) of the present invention is explained, referring to the figure(FIG. 2).

First, a Si substrate 102 is prepared. On the light-receiving side face(surface) of the Si substrate, electrodes (for example, electrodesmainly composed of Ag) 104 are installed (FIG. 2( a)). On the back faceof the substrate, aluminum pastes used as a back face electrode for asolar cell (although it is not particularly limited as long as it isused for a solar cell, for example, PV333, PV322 (commercially availablefrom E.I. du Pont de Nemours and Company)) 106 are spread by screenprinting, etc., then dried (FIG. 2( b)). The electroconductive paste ofthe present invention is then spread in a partially overlapped statewith the aluminum paste which was printed and dried in advance, anddried (FIG. 2( c)). The drying temperature of each paste is preferably150° C. or lower. Also, the aluminum paste preferably has a dried filmthickness of 40-60 μm, and the thickness of the silver/aluminumelectroconductive paste of the present invention is preferably 15-25 μm.Also, the overlapped part of the aluminum paste and the silver/aluminumelectroconductive paste is preferably about 0.5-2.5 mm.

Next, the substrate obtained is fired at a temperature of 700-950° C.for about 1-15 min, for instance, so that the desired solar cell isobtained (FIG. 2( d)). An electrode is formed from the composition(s) ofthe present invention wherein said composition has been fired to removethe organic medium and sinter the glass frit.

The solar cell obtained using the electroconductive paste of the presentinvention, as shown in FIG. 2( d), has electrodes 104 on thelight-receiving face (surface) of the substrate (for example, Sisubstrate) 102, Al electrodes 110 mainly composed of Al andsilver/aluminum electrodes 112 mainly composed of Ag and Al, on the backface.

EXAMPLES

Next, the present invention is explained in further detail by anapplication example. In the following application example, amanufacturing example of the silver/aluminum electroconductive paste ofthe present invention and an example in which the silver/aluminumelectroconductive paste is used as an electrode material for the backface of a Si substrate in manufacturing a solar cell are explained.

Application Example 1

Application Example 1 is detailed below.

The appropriate amount of organic medium, glass frit, inorganicadditives, and Ag/Al are mixed in a vacuum mixer for 15-30 min. Themixture was then sent through a power mixer or roll mill to disperse theingredients thoroughly. The degree of dispersion is measured by Finenessof Grind (FOG). When dispersion was sufficient, the paste was formulatedto the required solids and viscosity level.

Manufacture of Solar Cell

The present invention can be applied to a broad range of semiconductordevices, although it is especially effective in light-receiving elementssuch as photodiodes and solar cells. The discussion below describes howa solar cell was formed utilizing the composition(s) of presentinvention.

Using the silver/aluminum electroconductive paste obtained, a solar cellwas formed in the following sequence.

(1) On the back face of a Si substrate having a silver electrode on thefront surface, the newly invented Ag/Al paste is printed and dried.Typical dried thickness is in the range of 15 to 25 microns. Then analuminum paste for the back face electrode of a solar cell (for example,PV333, commercially available from E.I. du Pont de Nemours and Company)was screen-printed at a dried film thickness of 40-60 μm. The Ag/Al wasprinted as 5-6 mm wide bus bars, and the aluminum film overlaps with theAg/Al busbar for 1 mm at both edges to ensure electrical continuity. Insome applications, the entire backside may be covered with grid lines ofAg/Al, and in such situations, no Al paste will need to be printed.

(2) The printed wafers were then fired in a furnace with peaktemperature settings of 700 to 950 C for 1 to 10 minutes, depending onthe furnace dimensions and temperature settings. A solar cell was formedafter firing.

Test Procedure-Efficiency

The solar cells built according to the method described above wereplaced in a commercial IV tester for measuring efficiencies. The lightbulb in the IV tester simulated the sunlight with a known intensity andradiated the front surface of the cell, the bus bars printed in thefront of the cell were connected to the multiple probes of the IV testerand the electrical signals were transmitted through the probes to thecomputer for calculating efficiencies.

Test Procedure-Adhesion

After firing, a solder ribbon (copper coated with 62Sn/36Pb/2Ag) wassoldered to the bus bars printed on the backside of the cell. Soldercondition was typically at 345° C. for 5 seconds. Flux used was MF200.The soldered area was approximately 2 mm×2 mm. The adhesion strength wasobtained by pulling the ribbon at an angle of 90° to the surface of thecell. An assessment of the adhesion strength was assigned as low,adequate, or good, based on the assumption that an adhesion strengthless than 200 g is considered low; values in the range of 200 g to 300 gis adequate, values in the range of 300 to 400 or above is good.

Since the current invention comprises only Pb-free glass as the frit, wetested adhesion using both Pb-free solder and Pb-containing solder. ThePb-free solder used was 96.5Sn/3.5Ag. Solder temperature for the Pb-freesolder was in the range of 345-375° C., solder time was 5-7s. Flux usedwas MF200.

Criterion on bowing is as follows: >1 mm was considered high, 0.6-1 mmwas medium and <0.6 mm was considered low.

Example compositions are listed in Table 2. Glass Compositionsreferenced in Table 2 are detailed in Table 1. The performancecharacteristics of the examples are listed in Table 3.

TABLE 2 Compositions of Examples (Weight Percent Total Composition)Glass Glass Glass Glass Example Glass II III IV V Ag Al TiB2 Bi2O3 CuOrganic 1 5.12 68.98 2.47 0.20 23.23 2 5.12 68.98 2.47 0.20 23.23 3 5.1268.98 2.47 0.20 23.23 4 5.12 68.98 2.47 0.20 23.23 5 5.12 68.98 2.470.20 23.23 6 2.66 69.89 2.47 0.75 1.0 23.23

TABLE 3 Performance Characteristics of Example Compositions Adhesion (g)96.5 Sn/3.5 Ag Example 62 Sn/36 Pb/2 Ag Efficiency (%) Bowing (mm) 1 385± 76  401 ± 64 13.05 low 2 473 ± 98   714 ± 135 12.97 low 3 521 ± 117577 ± 95 13.67 low 4 427 ± 113 405 ± 44 13.61 high 5 80 ± 18 151 ± 33 NTNT 6 392 ± 66  NT 13.36 low

1. A thick film conductive composition comprising: (a) electroconductivemetal particles comprising a mixture of Ag particles and Al particles(b) glass frit wherein said glass frit is Pb-free; dispersed in (c) anorganic medium, wherein the average diameter of said electroconductivemetal particles is in the range of 0.5-10.0 μm, wherein said glass fritcomprises, in weight percent total glass fit composition: SiO₂ 0.5- 35,Al₂O₃ 0-5, B2O3 1-15, ZnO 0-15, and Bi₂O₃ 55-90, and further comprising:(d) one or more inorganic additives, wherein said inorganic additivesare selected from the group consisting of (1) Cu and Bi; (2) compoundsthat can generate elemental metals selected from Cu and Bi; (3) oxidesof Cu and Bi; and (4) mixtures thereof.
 2. The composition of claim 1further comprising one or more inorganic additives that are selectedfrom the group consisting of (1) TiB2, Ti, Al, Sn, Sb, Cr, Fe, Mn, Co,Ni, Ru, and B; (2) compounds that can generate elemental metals selectedfrom, Ti, Al, Sn, Sb, Cr, Fe, Mn, Co, Ni, Ru, B and; (3) oxides of, Ti,Al, Sn, Sb, Cr, Fe, Mn, Co, Ni, Ru, and B; and (4) mixtures thereof. 3.The composition of claim 1 comprising, based on weight percent totalcomposition: 40-93 weight percent Ag particles, 2-10 weight percent ofsaid glass frit, 1-5 weight percent Al particles and 5-50 weight percentorganic medium.
 4. the thick film composition of claim 1 wherein saidelectroconductive metal particles are in the form selected from (1)flakes, (2) spherical and (3) mixtures thereof.
 5. The thick filmcomposition of claim 1 wherein the softening point of said glass frit isin the range of 300°-550° C.
 6. An electrode formed from the compositionof claim 1 wherein said composition has been fired to remove the organicmedium and sinter said glass frit.
 7. A semiconductor device comprisingthe electrode of claim 6.